Electronic devices often have input terminals that receive signals of various types or output terminals from which output signals of various types are provided. In fact, many electronic devices have both such input terminals and output terminals. Such electronic devices include internal circuitry or components that may be damaged if a voltage having a large magnitude is applied to an input terminal or an output terminal. For example, electronic test equipment may have output terminals from which precision output voltages or currents are provided. The magnitude of these output voltages or currents may be relatively low, and the circuits or components that provide these output voltages or currents may be damaged if a relatively large voltage or current, such as an AC supply voltage, is applied to the output terminals.
It may seem to be a relatively simple matter to protect these circuits or components using, for example, a low current fuse. However, the circuits or components coupled to the terminals may be damaged before a fuse could reach melting temperature. Also, the impedance between the terminals may be too high to allow enough current to flow through the fuse responsive to a high voltage, so that the fuse would not open to protect the internal circuit or component. Fast-acting current sensing components might also be placed in parallel with the terminals. However, it may be important for all of the current supplied by an internal circuit or component to flow from the output terminal, thus precluding the use of a current sensing component in parallel with the output terminals which might draw current from the internal circuit or component that would otherwise flow from the output terminals. For example, the resistance of a circuit component connected between the terminals may be measured by coupling a specific current between the terminals and then measuring the voltage between the terminals. If current from a circuit supplying current to the terminals is diverted to a current sensing component coupled between the terminals, the resistance measurement may be in error.
It may therefore be important to be able to quickly decouple the external input or output terminals of an electrical device in a manner that does not draw current from or change the voltage between the input or output terminals.